Heretofore, in many power supplies, isolation transformers have been frequently used to protect electrical and electronic components or equipment from voltage transients or interference occurring on a power line. Such disturbances can be caused by switching large currents, lightning or other discharges on the supply lines. These disturbances are present both as common mode and transverse mode interferences.
To overcome common mode interference, in which the pair of power conductors both vary together in voltage and phase with respect to ground, the isolation transformers are shielded to prevent capacitive coupling between the primary and secondary windings. The shield is placed between the two windings and grounded, along with the core, so that common mode interference currents will flow to ground.
Such shielding, however, fails to effectively protect against interference produced by sudden transients occurring on one line with respect to the other in a transverse mode. This interference, usually of high frequency, is attenuated to only a limited degree by the transformer and requires further countermeasures as disclosed in U.S. Pat. Nos. 4,156,838 and 4,095,163 issued to H. R. Montague. The protection against transverse mode interference offered by these circuits includes an inductor in series with the power source and load and an auxiliary path in shunt with the load having parallel, bi-directional threshold devices in series with a capacitor. At normal powerline frequencies, the inductor and capacitor do not significantly load the powerline because the lowpass filter formed by these components has a cut-off frequency much higher than that of the power source. High frequency transients, on the other hand, see a low impedance path and the threshold devices provide a narrow dead band before allowing conduction of the transients.
The circuits shown in the two aforementioned patents have certain disadvantages. The series inductor can saturate thus limiting its effectiveness at high currents. Furthermore, there is energy inefficiency in the core losses of the inductor and there is the added cost of the manufacture and installation of the inductor. The inductor also has a limited high frequency response.